1. Field of the Invention
The present invention relates to a rotary solenoid and more specifically to a rotary solenoid configured to repeatedly rotate an output shaft within a specified angular range.
2. Description of the Related Art
A rotary solenoid for repeatedly rotating an output shaft within a specified angular range has heretofore been used as a drive power source for industrial machines. As the rotary solenoid of this kind, there is known, e.g., a so-called radial air-gap rotary solenoid in which a magnet and an electromagnetic coil are arranged in an opposing relationship along a rotation direction of an output shaft with a space left therebetween (see Japanese Patent No. 3240351). However, the radial air-gap rotary solenoid is required to become larger in size due to the arrangement of the electromagnetic coil or other causes. Thus, the radial air-gap rotary solenoid tends to become costly.
In recent years, there is proposed a so-called axial air-gap rotary solenoid in which a magnet and an electromagnetic coil are arranged along a direction parallel to an output shaft with a space left therebetween (see Japanese Patent Application Publication No. 2009-038874). In the axial air-gap rotary solenoid, two magnets juxtaposed to have different polarities are arranged in an axially opposing relationship with an electromagnetic coil having a major magnetic pole arranged at the center thereof. A swing yoke (back yoke) made of a magnetic material is arranged at the opposite side of the magnets from the electromagnetic coil with a space left therebetween. Thus, the axial air-gap rotary solenoid has a structure which is advantageous with respect to size reduction.
In the axial air-gap rotary solenoid, upon supplying an electric current to the electromagnetic coil, a swing unit is swung in one direction. The movement of the swing unit is restrained as a portion of the swing unit comes into contact with a stopper. If the electric current is supplied to the electromagnetic coil in a direction opposite to the aforementioned direction, the swing unit is swung in the reverse direction. The movement of the swing unit in the reverse direction is restrained as the swing unit comes into contact with another stopper. Accordingly, an output shaft can be repeatedly rotated within a specified angular range by controlling the supply of the electric current to the electromagnetic coil. If the electric current supplied to the electromagnetic coil is cut off, the swing unit is self-held by the magnetic attraction force of the magnets in the position where the swing unit makes contact with the stopper.
Depending on the use thereof, the rotary solenoid of this kind is required to have a suitable self-holding force and an operation speed characteristic while staying small in size. For that reason, an expensive rare-earth sintering material must be used as the magnet in most cases.
FIG. 17 is a section view of a conventional rotary solenoid having an axial air-gap structure, explaining a magnetic flux route in a self-holding state when no electric current is supplied. A housing 200 of a rotary solenoid includes an upper case 201, a lower case 202, and a side case 203 arranged to surround the periphery between the upper case 201 and the lower case 202. The upper case 201, the lower case 202, and the side case 203 are made of a magnetic material. An output shaft 204 is supported between the upper case 201 and the lower case 202. The output shaft 204 has an upper portion as an output end protruding beyond the upper case 201. Within the housing 200, a swing yoke 205 extending parallel to a plane orthogonal to the output shaft 204 is attached to the output shaft 204 in a position near the upper case 201. Two magnets 206 and 207 are attached to the tip end (free rotation end) of the swing yoke 205 by virtue of non-magnetic holders not shown. The magnets 206 and 207 are magnetized in the up-down direction parallel to the axial direction of the output shaft 204 and are juxtaposed so that the magnetic poles differing from each other can face downward. The magnets 206 and 207 are opposed to the swing yoke 205 across air gaps.
A ring-shaped electromagnetic coil 209 having a major magnetic pole 208 arranged at the center thereof is provided below the tip end of the swing yoke 205. The lower end portion of the major magnetic pole 208 is supported on and magnetically coupled to the lower case 202. Stoppers 210 and 211 facing each other along the swing direction of the swing yoke 205 are attached to the side case 203 in the height position corresponding to the magnets 206 and 207.
In the rotary solenoid of this configuration, if an electric current is supplied to the electromagnetic coil 209 in the state shown in FIG. 17 to thereby excite the major magnetic pole 208 so that the upper portion thereof can become an N-pole, a magnetic repulsion force is generated by one magnet 206. This is because the lower surface of one (right) magnet 206 facing the electromagnetic coil 209 is an N-pole. Thus, a swing force acts on the swing yoke 205, thereby swinging the swing yoke 205 so that one magnet 206 can move away from the major magnetic pole 208. In this swinging process, the repulsion force of one magnet 206 is gradually reduced along with the swinging movement of the swing yoke 205. On the other hand, the attraction force of the other (left) magnet 207 acting on the major magnetic pole 208 is gradually increased because the lower surface of the other magnet 207 is an S-pole. Thus, the swinging movement of the swing yoke 205 is continuously performed until a portion of the swing yoke 205 comes into contact with the stopper 210.
In the state that the other magnet 207 faces the major magnetic pole 208 of the electromagnetic coil 209, an electric current is supplied to the electromagnetic coil 209 in the direction opposite to the aforementioned direction to thereby excite the electromagnetic coil 209 so that the upper portion of the major magnetic pole 208 will become an S-pole. In this case, the swing yoke 205 is swung in the direction opposite to the aforementioned direction, thus coming into the state shown in FIG. 17. Even if the supply of the electric current to the electromagnetic coil 209 is stopped after finishing the swinging movement of the swing yoke 205, the swing position of the swing yoke 205 is kept as it is, because one of the magnets 206 and 207 magnetically attracts the major magnetic pole 208.
Arrows shown in FIG. 17 indicate the flow of magnetic flux. The magnetic path of the magnetic flux coming out from the N-pole of one magnet 206 includes: a route r1 in which the magnetic flux flows through the major magnetic pole 208, the lower case 202, and the side case 203 and then comes back to the S-pole of the other magnet 207 via an air gap; and a route r2 in which the magnetic flux entering the side case 203 flows from the upper case 201 toward the swing yoke 205 arranged above one magnet 206 and then comes back to one magnet 206 via an air gap.
In this case, each of the routes r1 and r2 has a long air gap. This makes it necessary for the magnetic flux to pass through the air gaps, which leads to an increased magnetic resistance. Moreover, air gaps exist between the swing yoke 205 and the magnets 206 and 207. Therefore, a large magnetic resistance is generated in these air gaps. A similar magnetic circuit is produced even when an electric current is supplied to the electromagnetic coil 209.
In the conventional configuration disclosed in Japanese Patent Application Publication No. 2009-38874, as described with reference to FIG. 17, the magnetic circuit of the rotary solenoid is formed through the use of the cases made of a magnetic material. Thus, the magnetic path becomes longer. In addition, the magnetic circuit suffers from a significantly large magnetic resistance and a reduced efficiency. For that reason, the grade of the magnet is increased or the thickness of the magnet is made unnecessarily large in order to obtain a specified characteristic. Further, the permeance modulus is increased to cope with demagnetization of the magnet. This poses a problem in that the material cost becomes too high and the size of the rotary solenoid grows large.